Various types of detectors, such as a thermal conductivity detector (TCD), an electron capture detector (ECD), a flame ionization detector (FID) and a flame photometric detector (FPD), have been put into practice as detectors for a gas chromatograph. From among these detectors, in particular, FIDs have been often used as a detector for detecting an organic substance. FIDs are a type of detector where sample molecules in a sample gas are ionized by a hydrogen flame and the ionization current is measured so as to provide a wide, dynamic range (see Patent Document 1). However, FIDs have such defects that a sufficiently low lower limit for detection cannot be gained due to the low efficiency in ionization, and in particular, the efficiency of ionization for alcohols, aromatics and chlorine-based substances is low, and furthermore, it is necessary to provide a special facility, such as an explosion-proof facility, because of its need for hydrogen, which makes handling troublesome.
In addition, pulse discharge detectors (PDD) have been known as a conventional detector that can detect, with high sensitivity, inorganic substances and organic compounds having a low boiling point. PDDs are a type of detector that excites helium molecules through a pulse discharge under high pressure, ionizes the molecules to be measured using light energy generated when the helium molecules in the excited state return to the ground state, and measures the ion current resulting from ionization. Accordingly, the PDD is easy to handle as compared to the FID because it does not use hydrogen. In addition, its efficiency of ionization is high as compared to that of the FID. However, the PDDs have such problems that the ionization is uneven and the dynamic range is narrow due to the instability of the plasma for ionization, and furthermore, a problem arises in that the electrodes and the like may be damaged due to high temperature plasma.
In order to solve these problems with the conventional detectors, a detector using a dielectric barrier discharge using low frequency, that is to say, a discharge ionization current detector or a detector that is referred to as a barrier discharge ionization detector (BID), has been proposed (see Patent Document 2).
FIG. 4 is a longitudinal cross-sectional diagram showing an example of the structure of a conventional discharge ionization current detector. One side of the tubular body 41 for forming the main body of the detector is formed of a crystal tube 42, and electrodes for discharge 43 to 45 are provided around the outer periphery of the crystal tube 42. An inlet for introducing a gas for generating plasma 46, through which a gas for generating plasma, such as helium, is introduced, is created at one end of the crystal tube 42. A bias electrode 47 and a collector electrode 48 are formed on the inner surface of the tubular body 41 on the side opposite to the crystal tube 42. In addition, an outlet for discharging a gas 49 is created in the sidewall of the tubular body 41 on the side opposite to the crystal tube 42 and further away than the collector electrode 48. In addition, a tubule 50 for introducing a sample gas to be measured is inserted through the other end of the tubular body 41. This tubule 50 is an end through which a sample gas flows out of the column in the case where the system is used as the detector for a gas chromatograph. Furthermore, this tubule (column end) 50 is connected to the tubular body 41 through a removable tube joint that can make tight sealing and connection possible, typically a ferrule joint 51.
In the above-described structure, a high voltage with low frequency is applied to the electrode 44 for discharge while introducing a gas for generating plasma into the tubular body 41 through the inlet for introducing a gas for generating plasma 46, and at the same time, the electrodes for discharge 43 and 45 are grounded so that a low frequency, alternating current exciting dielectric barrier discharge is generated and the gas for generating plasma is partially converted to plasma. In this state, a sample gas is introduced into the tubular body 41 through the tubule 50, and then, the component molecules of the sample gas are ionized due to the effects of the light released from the plasma or the excited species of helium. These ions are attracted to the bias electrode 47 to which a direct current bias voltage is applied, and furthermore are collected by the collector electrode 48, and thus, the ionization current is detected by a detector circuit that includes a current amplifier connected to the collector electrode 48.
The plasma under atmospheric pressure that is generated through the above-described low frequency, alternating current exciting dielectric barrier discharge is a non-equilibrium plasma where the neutral gas temperature is very low, and therefore, no such problems arise that an electrode is damaged due to the temperature, unlike the above, or the ionization becomes uneven due to the instability of the plasma.